(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a digital color copying machine and a printer, which forms a full-color image.
(2) Related Art
In a digital color copying machine which reproduces an image according to image data obtained by reading an original with a scanner, digital image data of red (R), green (G), and blue (B) obtained by performing color-separation on an original image, are transformed into digital image data of cyan (C), magenta (M), and yellow (Y) used for image reproduction.
During the data processing, however, image noise is caused by low scanning accuracy, poor color reproducibility of the printer, and other factors, and that is why a correction unit is needed to perform corrections on the image data.
In a full-color reproduction image, unevenness is very noticeable in a uniform density region of a chromatic color, where smoothness is very important as well as resolution. However, if resolution is emphasized, roughness stands out even more in the uniform density region and makes the whole image unnatural, and if smoothness is emphasized, the edge portion is blurred and results in very poor resolution.
Japanese Laid-open Patent Application No. 4-342370 discloses an image processing method, in which the edge portions and the uniform density regions are discriminated based on changes in lightness image data. Here, the edge portions are subjected to edge-emphasizing to reproduce a clearer image, while the uniform density regions are subjected to smoothing to obtain a smoother image. By doing so, both resolution and smoothness are improved, and image noise is reduced.
According to this conventional image processing method, however, reproduction images tend to be unnatural because smoothing is performed with uniform intensity on the uniform density regions.
Image defects which are particularly noticeable are roughness and noise in the facial skin color. If the smoothing is intensified to eliminate roughness and noise, other uniform density regions which do not need smoothing much are also subjected to the intensified smoothing. As a result, the overall resolution becomes lower, making the reproduction image even more unnatural.